FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to an electrostatic charge emitting apparatus particularly suitable for use in image forming devices such as electrophotographic copy machine and electrophotographic printer.
In the electrophotographic copy machine or the electrophotographic printer, a plurality of corona chargers are used for electrifying a surface of an image forming member such as a photosensitive drum, for transferring a toner image formed on the photosensitive drum to a recording paper, and for de-charging the charged photosensitive drum in order to separate the recording paper therefrom.
The corona charger mentioned above comprises a corona wire mounted in a shield case; and the wire is connected to a DC power supply. When a voltage of about 6 KV is applied to the wire, an electric field is generated in a space surrounding the corona wire in the shield case; the space is ionized due to the high voltage applied to the corona wire and the ion is discharged toward the image forming member by the force due to the electric field; and a surface of the image forming member such as the photosensitive drum is uniformly charged. Hereinafter, an electrode for generating the electric field such as the corona wire is also represented by a discharging member. However, in the known corona chargers, the corona wire having a diameter of about 60 .mu.m, which is relatively thick, is used and it is necessary to make the distance between the corona wire and the shield case long. To this end, the high electric voltage such as 6 KV has to be applied to the wire and therefore an extremely great amount of ozone gas, which affects the human body, is generated from the corona charger. Thus, it has been required to develop a charge emitting apparatus in which ozone gas is not generated so much.
In order to meet this requirement, a brush charging method has been proposed. In this method, a brush roller on which a plurality of conductive fibers each having a diameter of about 10 .mu.m are planted is used; the brush roller is arranged such that the tip portions of the fibers are made slidably contact with the surface of a photosensitive image forming member; and the photosensitive image forming member is charged by the corona discharge generated at the tip portions of the brushes. In this method, since it is possible to reduce the diameter of the conductive fiber to about 10 .mu.m, the threshold voltage at which discharging starts becomes low and thus the generation of ozone gas can be decreased.
However, this method has some practical disadvantages such that since the conductive fibers planted in the brush roller are slidably contacted with the surface of the photosensitive image forming member, a mechanical stress is strongly effected to the fibers. As a result, in case that the brush is used for a long time, the tip portions of the fibers is transformed so as not to be elastically recovered due to the mechanical stress or the fibers are fallen off. To this end, the tips of the fibers would become not to contact with the photosensitive member evenly, so that the whole surface of the photosensitive image forming member cannot to be charged uniformly. Also, in order to charge the surface of the photosensitive image forming member evenly, it is necessary to strictly control the distance between the tip portions of the fibers and the surface of the photosensitive member. If the fibers are contacted with the photosensitive member strongly, the tips of the fibers will be bent largely, and therefore the tips of the fibers are separated from the surface of the photosensitive image forming member so largely that the electric potential on the surface of the member will be decreased. While, if the contacting amount of the fibers is small, the corona discharge is not generated unequally, so that the electric potential on the surface of the member becomes uneven. Therefore, the setting operation of the brush roller to the surface of the photosensitive image forming member is so complicated. Further, in case the brush roller is used for a long time, a toner on the photosensitive image forming member which has not removed therefrom in a transferring process is adhered to the surface of the brush. As the result, no corona discharge is locally generated, so that the photosensitive image forming member cannot be charged uniformly.